Process for producing caprolactam

ABSTRACT

A process for preparing caprolactam by reacting 6-aminocapronitrile with water in the presence of catalysts comprises using a starting mixture of 6-aminocapronitrile and the tetrahydroazepine derivative of the formula                    
     and conducting the reaction in liquid phase in the presence of a heterogeneous catalyst. Also describes a process for preparing said tetrahydroazepine derivative I and its use for preparing caprolactam and polycaprolactam.

The present invention relates to an improved process for preparingcaprolactam by reacting 6-aminocapronitrile with water in the presenceof catalysts.

On heating or storage at room temperature, 6-aminocapronitrile forms abrown tetrahydroazepine derivative (THA derivative I) of the formula

THA derivative I shall also encompass its tautomeric form

EP-A-497,333 describes the direct polymerization of polycaprolactamstarting from 6-aminocapronitrile. The problem to be solved in theprocess mentioned was the removal of tetrahydroazepine (“THA”) beforethe polymerization step, since tetrahydroazepine leads to discolorationof the polymer obtained on polymerizing caprolactam in the presence oftetrahydroazepine. EP-A-497,333 proposes solving the problem by means ofa treatment with a basic compound such as an alkali metal hydroxide oran alkali metal alkoxide. Following the treatment, 6-aminocapronitrilecan be conveniently separated from the reaction mixture by distillation,which is not possible without such a treatment. EP-A 502,439 solves theproblem of removing THA in the presence of 6-aminocapronitrile bytreatment with sodium borohydride. Here too 6-aminocapronitrile can bereadily separated from the reaction mixture by distillation after thetreatment.

DE-B-25 42 396 and DE-B-25 42 397 describe the conversion ofgamma-aminobutyronitrile into a mixture comprising2-(N-gamma-cyanopropyl)amino-deltal-pyrroline (“CAP”) and2-amino-deltal-pyrroline (“AP”), and also the further hydrolysis of theisolated CAP to 2-pyrrolidone in the absence of catalysts. Neitherreference indicates whether the corresponding THA derivative I can beconverted into caprolactam in a similar manner in liquid phase in thepresence of heterogeneous catalysts. Furthermore, in the cited DEreferences CAP is first isolated as a pure substance before it ishydrolyzed. It might therefore be expected that the use of mixturescomprising THA derivative I would promote the formation of undesirableby-products. It is also known that five-membered rings are easier toform than seven-membered rings (see Römpp Chemie Lexikon, 9th edition,editors Falbe and Regitz, Georg Thieme verlag, New York). Altogether andon the basis of experience with THA it might therefore be expected thatTHA derivative I would lead to discolored caprolactam in the cyclizationof 6-aminocapronitrile and to discolored polycaprolactam in the directconversion of 6-aminocapronitrile into polycaprolactam, unless separatedoff before the cyclization and before the polymerization step.

It might further be expected that THA derivative I would reduce thelifetime of the catalyst used in the polymerization, since it was knownfrom U.S. Pat. No. 5,162,567 that heating THA produces high boilers, ie.compounds or mixtures with a higher boiling point than6-aminocapronitrile (accordingly making it easy to remove the6-aminocapronitrile). High boilers, however, tend to form polymeric oroligomeric decomposition products which can form deposits on catalystsurfaces and so reduce not only the lifetime but also the activity ofthe catalysts.

It is an object of the present invention to provide a process forcyclizing 6-aminocapronitrile to caprolactam wherein THA derivative Ireduces neither the lifetime nor the activity of the cyclizationcatalyst, nor leads to a caprolactam-containing reaction mixture whoseUV number is equal to or higher than that prior to the cyclization step.Preferably the post-cyclization UV number should be smaller thanpre-cyclization as a function of the pre-cyclization THA derivative Icontent. Furthermore, any THA derivative I present in the reactionmixture for the direct polymerization of 6-aminocapronitrile shall beeasy to remove or it shall be possible to conduct the reaction in such away that THA derivative I is eliminated.

We have found that this object is achieved by a process for preparingcaprolactam by reacting 6-aminocapronitrile with water in the presenceof catalysts, which comprises using a starting mixture of6-aminocapronitrile and the tetrahydroazepine derivative of the formula

and conducting the reaction in liquid phase in the presence of aheterogeneous catalyst.

The present invention also provides tetrahydroazepine derivative I, aprocess for its preparation, and the use of THA derivative I forpreparing caprolactam.

The reaction of the present invention is carried out in liquid phase inthe presence of heterogeneous catalysts at temperatures from generally140 to 320 ° C., preferably from 160 to 280° C.; the pressure isgenerally within the range from 41 to 250 bar, preferably from 5 to 150bar, care having to be taken to ensure that, under the conditionsemployed, the reaction mixture is predominantly (ie. without thecatalyst, which is present in solid phase) liquid. The residence timesare generally within the range from 1 to 120, preferably from 1 to 90,in particular from 1 to 60, min. In some cases residence times from 1 to10 min will prove completely adequate.

The amount of water used is generally at least 0.01 mol, preferably from0.1 to 20 mol, in particular from 1 to 5 mol, per mole of THA derivativeI.

Advantageously THA derivative I is used in the form of a from 1 to 50%strength by weight, in particular from 5 to 50% strength by weight,particularly preferably from 5 to 30% strength by weight, solution inwater (in which case the solvent is then also the reactant) or inwater-solvent mixtures. Examples of suitable solvents are alkanols suchas methanol, ethanol, n- and i-propanol, n-, i- and t-butanol andpolyols such as diethylene glycol and tetraethylene glycol, hydrocarbonssuch as petroleum ether, benzene, toluene, xylene, lactams such aspyrrolidone or caprolactam or alkyl-substituted lactams such asN-methylpyrrolidone, N-methylcaprolactam or N-ethylcaprolactam and alsocarboxylic esters, preferably of carboxylic acids having from 1 to 8carbon atoms. Ammonia too can be present in the reaction. It is ofcourse also possible to use mixtures of organic solvents. Mixtures ofwater and alkanols in a water:alkanol weight ratio of 1-75:25-99,preferably 1-50:50-99, have been determined to be particularlyadvantageous in some cases.

The THA derivative I content in the 6-aminocapronitrile of the startingmixture can be within the range from 0.01 to 95% by weight, inparticular from 0.1 to 50% by weight, particularly preferably from 0.5to 20% by weight.

The starting mixture customarily has, depending on the level of THAderivative I, a UV number (sum of all absorbances of a 10% by weightsolution in ethanol at wavelengths from 280 to 400 nm, based on a pathlength of 5 cm) within the range from 5 to 40,000.

The starting mixture is obtainable by heating 6-aminocapronitrile withor without solvent. From experience to date, the temperature can bewithin the range from 20 to 280° C., in particular within the range from50 to 250° C., particularly preferably within the range from 100 to 230°C. The reaction times are customarily within the range from 10 minutesto 20 hours. As expected, shorter reaction times are possible at highertemperatures. The reaction can be carried out at pressures within therange from 100 kPa to 25 MPa, preferably from 500 kPa to 20 MPa. It canfurther be advantageous to carry out the reaction in the presence ofacidic homogeneous or heterogeneous catalysts such as mineral acid,carboxylic acids, sulfonic acids, titanium dioxide, aluminum oxide, acidion exchangers or Lewis acids.

If desired, pure THA derivative I can be obtained for example bydistillation of unconverted 6-aminocapronitrile, solvents and anyby-products.

Examples of suitable heterogeneous catalysts include: acidic, basic oramphoteric oxides of the elements of the second, third or fourth maingroup of the periodic table, such as calcium oxide, magnesium oxide,boron oxide, aluminum oxide, tin oxide or silicon dioxide as pyrogenicsilica, as silica gel, diatomaceous earth, quartz or mixtures thereof,also oxides of metals of secondary groups two to six of the periodictable, such as titanium oxide, amorphous, as anatase and/or rutile,zirconium oxide, zinc oxide, manganese oxide or mixtures thereof. It isalso possible to use oxides of the lanthanides and actinides, such ascerium oxide, thorium oxide, praseodymium oxide, samarium oxide, rareearth mixed oxide, or mixtures thereof with the aforementioned oxides.Further catalysts can be, for example:

vanadium oxide, niobium oxide, iron oxide, chromium oxide, molybdenumoxide, tungsten oxide or mixtures thereof. Mixtures between the oxidesmentioned are also possible. It is also possible to use sulfides,selenides and tellurides such as zinc telluride, tin selenide,molybdenum sulfide, tungsten sulfide, sulfides of nickel, of zinc and ofchromium.

The aforementioned compounds may be doped, ie. contain, compounds ofmain groups 1 and 7 of the periodic table.

Also suitable are zeolites, phosphates and heteropolyacids and alsoacidic and alkali iron exchangers such as, for example, Naphion®.

If desired, these catalysts may contain up to 50% by weight each ofcopper, tin, zinc, manganese, iron, cobalt, nickel, ruthenium,palladium, platinum, silver or rhodium.

The catalysts can be used as solid catalyst or supported catalyst,depending on the composition of the catalyst. For instance, titaniumdioxide can be used in the form of a titanium dioxide extrudate or inthe form of a thin layer applied to a carrier. Any method described inthe literature is suitable for applying TiO₂ to a carrier such assilicon dioxide, aluminum oxide or zirconium dioxide. For instance, athin layer of TiO₂ can be applied by hydrolysis of organotitaniums suchas titanium isopropoxide or titanium butoxide, or by hydrolysis of TiCl₄or other inorganic Ti-containing compounds. Sols which contain titaniumdioxide are also suitable.

Particular preference is given to catalysts which contain noconstituents that are soluble under the conditions of the reaction.

In a further preferred embodiment, the reaction is carried out in afixed bed reactor. A fixed bed process is customarily carried out withtablets or extrudates having diameters within the range from 1 to 10 mm.In principle, however, the reaction can also be carried out insuspension.

In a further preferred embodiment, the reaction is carried out inparticular in the presence of a heterogeneous catalyst based on titaniumdioxide, zirconium dioxide, cerium oxide or aluminum oxide.

Aluminum oxide is generally suitable in all modifications obtained byheating the precursor compounds aluminum hydroxide (gibbsite, boehmite,pseudoboehmite, bayerite and diaspore) at different temperatures. Theseinclude in particular gamma- and alpha-alumina and mixtures thereof.

The oxides can be used in pure form (purity of the respective oxide>80%by weight), as a mixture of the abovementioned oxides, in which case thesum of the abovementioned oxides should be>80% by weight, or assupported catalyst, in which case the abovementioned oxides can beapplied to a mechanically and chemically stable carrier usually with ahigh surface area.

The pure oxides can be prepared by precipitation from aqueous solutions,for example titanium dioxide by the sulfate process or by otherprocesses such as the pyrogenic production of fine alumina, titania orzirconia powders which are commercially available.

Mixtures of various oxides can be prepared in various ways. The oxidesor their precursor compounds, which are convertible into the oxides bycalcination, can be prepared for example by coprecipitation fromsolution. This generally brings about very good dispersion of the twooxides used. The oxide or precursor mixtures can also be precipitated byprecipitating one oxide or precursor in the presence of a finesuspension of the second oxide or precursor. A further method consistsin mechanically mixing the oxide or precursor powders, this mixture canbe used as a starting material for producing extrudates or tablets.

Supported catalysts can be prepared by customary methods. For instance,the oxides can be applied to the support by simply impregnating thesupport with their sols. The sol volatiles are customarily removed fromthe catalyst by drying and calcining. Sols of this type are commerciallyavailable for titania, alumina and zirconia.

A further way of applying layers of the active oxides is the hydrolysisor pyrolysis of organic or inorganic compounds. For instance, a ceramicsupport can be coated with a thin layer of titanium dioxide byhydrolysis of titanium isopropoxide or other titanium alkoxides. Othersuitable compounds include TiCl₄, zirconyl chloride, aluminum nitrateand cerium nitrate. Suitable supports are powders, extrudates or tabletsof the aforementioned oxides themselves or of other stable oxides suchas silica. The supports used can be macroporous to improve the masstransport.

In a further particularly preferred embodiment, the catalyst used istitanium dioxide with an anatase content within the range from 100 to 5,preferably from 99 to 10%, by weight and a rutile content within therange from 0 to 95, preferably from 1 to 90%, by weight, based on thetotal amount of titanium dioxide. THA derivative I is preferably usedfor preparing caprolactam by heating it with water/solvent at atemperature within the range from 140 to 320° C., preferably within therange from 160 to 280° C., and a pressure within the range from 100 to2500, in particular within the range from 500 to 2000, kPa in thepresence of the abovementioned heterogeneous catalysts, preferablytitania-containing, similarly to the abovementioned starting mixture,using a molar ratio of tetrahydroazepine derivative I to water withinthe range from 0.01:1 to 20:1, preferably from 0.5:1 to 20:1.

The abovementioned starting mixture as aqueous solution and THAderivative I alone can be directly converted into polycaprolactam byheating by known methods, for example described in EP-A-150,295.

The advantage of the process of the present invention is that itprovides a convenient way of processing THA-derivative-I-containingreaction mixtures with 6-aminocapronitrile into caprolactam and, ifdesired, into polycaprolactam. The products and product mixtures thusobtained are free of troublesome THA derivative I. Thus there is no needfor further process steps in the use of additional agents, compared withthe removal of THA from corresponding reaction mixtures.

In certain circumstances it can even be advantageous to convert6-aminocapronitrile in whole or in part into THA derivative I bypreheating to temperatures from 20 to 280° C. and to use the resultingmixture of THA derivative I and 6-aminocapronitrile for the cyclizationover oxidic catalysts.

EXAMPLES Example 1

400 g of 6-aminocapronitrile (ACN) were heated to 200° C. for 8 h.

Distillation yielded as second fraction at 0.1 mbar and 140° C. 40 g ofTHA derivative I (yield 10%) as a pure compound. The characterizationwas carried out by means of NMR spectroscopy:

¹H-NMR (250 MHz, DMSO-d₆, TMS, ppm): 4.2 (s, broad, 1 H), 3.2 (m, 2H),2.9 (t, 2H), 2.45 (t, 2H), 2.25 (m, 2H), 1.7-1.1 (m, 12 H). ¹³C-NMR(62.9 MHz, DMSO-d₆, TMS, ppm): 163.3 s, 120.6 s, 47.0 t, 41.6 t, 32.9 t,30.6 t, 29.7 t, 28.4 t, 26.0 t, 25.6 t, 24.8 t, 16.2 t.

Example 2

A 10% by weight ethanolic solution of THA derivative I was pumpedtogether with 2 mol of water (corresponding to 3.2% by weight of thetotal solution) through a titania-packed tubular reactor (diameter 6 mm;length 800 ml at 70 ml/h. The reactor temperature was 230° C., thepressure was 80 bar. The hourly output was a 9.7% ethanolic caprolactamsolution. The solution further obtained 10.8% by weight of recyclableethyl 6-aminocaproate and also 0.2% by weight of recyclable6-aminocapronitrile. The caprolactam yield was 80%, the selectivityincluding the recyclable compounds was 95%.

Example 3

A 10% by weight ethanolic solution consisting of 95% by weight of ACNand 5% by weight of THA derivative I was pumped together with 2 mol ofwater (corresponding to 3.2% by weight of the total solution) through atitania-packed tubular reactor (diameter 6 mm; length 800 ml at 70 ml/h.The reactor temperature was 230° C., the pressure was 80 bar. The hourlyoutput was a 9.1% ethanolic caprolactam solution. The solution furthercontained 0.4% by weight of recyclable ethyl 6-aminocaproate and also0.1% by weight of recyclable 6-aminocapronitrile. The caprolactam yieldwas 91%, the selectivity including the recyclable compounds was 95%.

Example 4

A 10% by weight ethanolic solution consisting of 99% by weight of ACNand 1% by weight of THA derivative I was pumped together with 2 mol ofwater (corresponding to 3.2% by weight of the total solution) through atitania-packed tubular reactor (diameter 6 mm; length 800 ml at 70 ml/h.The reactor temperature was 230° C., the pressure was 80 bar. The hourlyoutput was a 9.0% ethanolic caprolactam solution. The solution furthercontained 0.4% by weight of recyclable ethyl 6-aminocaproate and also0.1% by weight of recyclable 6-aminocapronitrile. The caprolactam yieldwas 90%, the selectivity including the recyclable compounds was 95%.

We claim:
 1. A process for the preparation of caprolactam by reacting6-aminocapronitrile with water, which process comprises reacting amixture of 6-aminocapronitrile and the tetrahydroazepine of the formula(I)

which mixture comprises at least 0.01% by weight of thetetrahydroazepine of the formula I, in the liquid phase in the presenceof a heterogeneous catalyst.
 2. The process defined in claim 1, which iscarried out in a fixed bed reactor.
 3. The process defined in claim 1,wherein the heterogeneous catalyst is based on titanium dioxide,zirconium dioxide, cerium oxide or aluminum oxide.
 4. The processdefined in claim 1, wherein the heterogeneous catalyst comprises of from5 to 100% by weight of anatase and of from 0 to 95% by weight of rutile,based on the total amount of titanium dioxide.
 5. The process defined inclaim 1, wherein the mixture of 6-aminocapronitrile and thetetrahydroazepine of the formula (I) has a UV number (sum of allabsorbances of a 10% by weight solution in ethanol at wavelengths from280 to 400 nm, based on a pathlength of 5 cm) within a range from 5 to40,000.
 6. The process defined in claim 1, wherein the mixture of6-aminocapronitrile and the tetrahydroazepine of the formula (I)comprises of from 0.01 to 95% by weight of 6-aminocapronitrile.
 7. Theprocess defined in claim 1, wherein the tetrahydroazepine of the formula(I) is used as a 1 to 50% strength by weight aqueous solution.
 8. Theprocess defined in claim 1, which is carried out in the presence of anadditional solvent.